In the food industry, the chemical industry, and the pharmaceutical industry fields, for example, a type of evaporator called “falling film evaporator” is used to recover a solvent from a liquid containing foreign substances and impurities or to concentrate the liquid.
FIG. 13 is a diagram schematically showing an evaporation system including a conventional falling film evaporator.
An evaporation system 900 includes a raw material tank 910 containing a raw material liquid serving as a raw material, a falling film evaporator 800, a vacuum pump 920, and a condenser 930. The raw material liquid is caused to flow from the raw material tank 910 to a preheater 906 through a conduit 904 by driving of a pump 902, temporarily preheated in the preheater 906, and then fed to the evaporator 800.
FIG. 14 is a diagram schematically showing a portion of a cross section of the evaporator 800 constituting the evaporation system shown in FIG. 13.
As shown in FIG. 14, the evaporator 800 includes an agitation vessel 810, a rotating shaft 821 extending in the vertical direction within the agitation vessel 810 and being rotatable in the horizontal direction, a plurality of supports 822 individually extending in the horizontal direction from the rotating shaft 821 in an upper portion and a lower portion of the agitation vessel 810, and rollers 826 extending downward from the respective supports 822 and being provided so as to come into contact with an inner wall of the agitation vessel 810. The rotating shaft 821 is connected to a driving motor portion 840.
A raw material liquid 834 fed from the raw material tank is supplied to an upper portion of the inner wall of the agitation vessel 810 through supply ports 832 extending in the horizontal direction from the rotating shaft 821, while being rotated as per driving of the driving motor portion 840. After that, the raw material liquid 834 flows downward along the inner wall of the agitation vessel 810 while forming a wet surface. On the other hand, an outer circumference of the agitation vessel 810 is covered by a jacket 812 that can be heated by steam, for example. Here, when the jacket 812 is heated, the heat is transferred from the outer circumference to the inner wall of the agitation vessel 810 and causes a volatile component contained in the raw material liquid 934 flowing down the inner wall while forming a wet surface to evaporate. The evaporated volatile component is fed to the condenser 930 (FIG. 13) provided outside the evaporator 800 through a vapor outlet 860. The volatile component is cooled in the condenser 930, then returns to the liquid state, and is finally collected as a condensate. On the other hand, in FIG. 14, components contained in the raw material liquid other than the above-described volatile component flow down the inner wall of the agitation vessel 810 as is, and are discharged to the outside of the evaporator 800 through discharge ports 880 provided in a bottom portion of the agitation vessel 810.
While the raw material liquid flows down within the agitation vessel 810 as described above, the driving motor portion 840 drives the rollers 826 provided on the respective supports 822 to circle along the inner wall of the agitation vessel 810 while being in contact therewith.
FIG. 15 is a diagram schematically showing a cross section of the conventional evaporator 800 shown in FIG. 14 taken in the direction A-A′. In the evaporator 800, the rollers 826 are in contact with and circle along the inner wall of the agitation vessel 810 heated by the jacket 812, thereby forcibly subjecting the raw material liquid present on a heat transfer surface of the inner wall to surface renewal, and thus, the evaporation efficiency can be increased. Although FIG. 15 shows that the rollers 826 are provided, there are also conventional evaporators in which wipers are provided instead of the rollers 826.
However, some matters of concern have been pointed out with respect to such evaporators.
One of those matters is that the supplied raw material liquid passes down the inner wall (heat transfer surface) within the agitation vessel by flowing down only once through a so-called “one path”. Also, in the case where the raw material liquid contains a large amount of a volatile component or in the case where the volatile component cannot be sufficiently evaporated while the raw material liquid flows down the inner wall, it is considered that the remaining component is discharged through the discharge ports 880 as is. For this reason, it has been recognized that the use of the above-described evaporator for a raw material liquid that is required to be sufficiently concentrated is difficult.
Moreover, the rollers 826 such as those shown in FIG. 15 or wipers are continuously in contact with the heat transfer surface and are thus likely to wear out. For this reason, regular replacement is required, and it has been pointed out that the working hours, labor, and costs for maintenance increase accordingly.
Furthermore, it has been pointed out that, in the case where the evaporator is to be stopped, since the temperature of the inner wall is higher than the liquid temperature, if the supply of the raw material liquid is stopped as is, the rollers or the wipers, which are in contact with the inner wall, will deform or deteriorate due to high heat. For this reason, when the evaporator is to be stopped, it is necessary to continue the supply of the raw material liquid or the circulation of a product within the agitation vessel until the temperature of the inner wall decreases.